PROJECT SUMMARY/ABSTRACT Adult onset neuronal ceroid lipofuscinosis (ANCL) is an invariably fatal lysosomal storage disease with no treatment and no known mechanism of pathogenesis. The objective of proposed studies is to delineate the mo- lecular mechanism of ANCL caused by mutations in cysteine string protein-? (CSP?), and then pharmacologi- cally rescue the molecular dysfunction. Recently, several studies have found ANCL-causing mutations in the gene encoding CSP?. Yet, exactly how ANCL mutations in CSP? lead to lysosomal dysfunction and neuron death remains unclear. Previously, CSP? was shown to chaperone the synaptic SNARE protein SNAP-25. We have now found that the SNAP-25 homolog SNAP-23 is also chaperoned by CSP?. Importantly, SNAP-23 is a SNARE protein which mediates lysosomal exocytosis, offering a direct connection from CSP? dysfunction to lysosomal pathology. We have obtained exciting preliminary data showing: a) ANCL mutations in CSP? disrupt its palmitoylation and membrane association, and thus its binding to SNAP-23; b) In CSP?-knockout mouse brains and neurons, there is significant reduction in SNAP-23 protein levels, in lysosomal SNARE-complex as- sembly, and in Ca2+-dependent lysosomal exocytosis; c) CSP?-knockout mouse brains reveal premature accu- mulation of lipofuscin followed by neurodegeneration; and d) Knockdown of SNAP-23 (not of SNAP-25) re- duces lysosomal exocytosis and leads to lipofuscin accumulation in WT neurons. These data have led to the central hypothesis that ANCL mutations in CSP? disrupt its chaperoning of SNAP-23, causing disruption of ly- sosomal exocytosis, and leading to lipofuscinosis in ANCL, and these defects can be rescued by chemical chaperones. To test this hypothesis, three specific aims are proposed: Aim 1. Determine how ANCL mutations in CSP? affect SNAP-23 stability and lysosomal exocytosis. Aim 2. Determine how ANCL mutations in CSP? lead to lysosomal pathology and neurodegeneration, in mouse brains in vivo and in ANCL patient-derived in- duced-neurons (iNs). Aim 3: Identify pharmacological strategies to rescue the SNARE and lysosomal defects. This proposal is innovative in introducing a) a new neuroprotective function of CSP? in lysosomal exocytosis, via chaperoning a new client protein, SNAP-23; b) a new concept that defects in lysosomal exocytosis can contribute to lysosomal storage pathology; c) using a multidisciplinary approach including new models of ANCL (e.g. patient-derived iNs with syngeneic wild type iNs generated via CRISPR/Cas9 gene-editing, and CSP?-/- mice lentivirally expressing ANCL mutants CSP?L115R or CSP?L116del in vivo). This proposal is significant be- cause completion of these studies is expected to delineate the pathological cascade of ANCL at the molecular level (Aim 1) and in vivo (Aim 2), and to evaluate pharmacological agents with potential to correct the molecu- lar defect(s) (Aim 3). At the same time, this work will reveal some of the fundamental mechanisms and func- tions of lysosomal exocytosis in neurons.